Vibration has an adverse affect on the productivity of work vehicles used in agriculture and construction in which an operator sits in a seat of a cab that is supported on a chassis. As the vehicle moves over the ground, vibration disturbances are transmitted through the vehicle components to the seat and the operator. The motion disturbances experienced by such vehicles increase operator fatigue and can result in considerable operator discomfort over sustained working periods. Thus, it is desirable to reduce operator movement by minimizing motion disturbances that otherwise are transmitted to the seat.
Motor vehicles have traditionally incorporated devices into suspension systems between the wheels and the vehicle chassis to attenuate vibration induced by the road. Additional vibration isolation devices sometimes are placed between the vehicle chassis and the cab in which the operator rides to further reduce road vibrations and engine induced vibration. Those previous suspension systems typically performed poorly in the frequency range where the human body is most sensitive, i.e. one to ten hertz. When subjected to vertical movement, or bounce, the human abdomen resonates at approximately four to eight hertz while the head and eyes resonate at ten hertz. The upper torso of a person resonates in response to pitch and roll movement at between one and two hertz. As a consequence, the vehicle suspension system needs high performance at these frequencies and directions in order to counteract vibration effectively.
Very soft cab mounts can provide attenuation in this low frequency range (one to ten hertz), but have very poor force rejection ability. In other words, a relatively small external force applied to the cab causes the vehicle cab to deflect unacceptably. Other vehicle suspension systems, which are relatively stiff and thus have good force rejection, tend to provide poor low frequency isolation. In many instances, such systems actually amplify the frequency range to which the human body is most sensitive.
It is more desirable to have a suspension system which is hard relative to external forces acting on the vehicle body, but soft to disturbances transferred from the chassis up to the cab, in other words, a hard/soft system. With such a system, the body feels rigid when the operator climbs into the vehicle, but the offending vibrations which would otherwise be transmitted from the chassis to the body are attenuated.
With reference to FIG. 1, a conventional passive suspension system for a vehicle consists of a spring and a damper 7, such as a conventional shock absorber, connected in parallel between the chassis 8 and body 9 of the vehicle. The motion of the body is defined by the expressions: EQU P=K.delta.+R(V.sub.I -V.sub.0) EQU .delta.=V.sub.I -V.sub.0
The transmissibility of the suspension is given by: ##EQU1## where M is the mass of the body, K is the stiffness of spring 6, R is the damping coefficient R of the damper 7, V.sub.0 is inertial velocity of the body mass, V.sub.I is the inertial velocity of the chassis disturbance, and s is the Laplace variable.
A trade-off exists in the design of this simple spring and damper suspension system. In order to isolate vibrations at relatively high frequencies above the suspension's natural frequency, it is desirable to reduce the damping coefficient R. However, such a system tends to resonate, as an automobile with badly worn and ineffective shock absorbers, thereby producing a very springy ride. Increasing the damping coefficient to overcome the springy ride problem decreases isolation above the suspension's natural frequency.
Previous attempts to avoid this trade-off employed a dynamically altered control force applied across the suspension. In that system the force Fc exerted by the damper varied in proportion to the mass velocity. The motion of the mass in that system is defined by the expressions: EQU p=K.delta.+RV.sub.0 EQU .delta.=V.sub.I -V.sub.0
Thus the dependence on the motion of the chassis has been removed. The transmissibility of the suspension is given by: ##EQU2##
As evident from the above transmissibility function, the feedback from the sensed mass motion affects only the damping term in the denominator, eliminating the trade-off between dampening at resonance and isolation above the suspension's natural frequency.